Despite the substantial progress made heretofore in the technology of die attach adhesives, there remains a very real need for such adhesives which can not only be applied on line in a very rapid manner, but also at lower temperatures. Furthermore, these new adhesives must be able to withstand higher wire bonding temperatures and be more resistant to moisture.
In the mass production of integrated chip assemblies, a particularly critical step to productivity of the process is the step of bonding the integrated circuit (IC) chip to its substrate. Such substrates include lead frames, pre-molded ceramic packages, pre-molded plastic packages, ceramic substrates and printed wiring boards. Heretofore, this has been done either by bonding the IC chip by means of a eutectic process, inorganic binder or by means of a filled epoxy adhesive. In the latter two cases, the assembly must be removed from the assembly line and processed further in some way. For example, in the case of inorganic binders, the assembly must be fired off-line to effect sintering of the inorganic binder and bonding of the IC chip to the substrate, thus removing the part from the assembly sequence for up to two hours typically and frequently longer. Similarly, in the case of filled epoxy binders, the assembly must be removed from the assembly sequence to allow curing (crosslinking) of the epoxy resin, which may require from several minutes to as much as an hour for completion.
From the foregoing description of the current state of the art, it is evident that there is a very substantial unmet need for a method of bonding IC chips to substrates by which appropriate adhesion of the IC chip to the substrate can be achieved so quickly that "in-line" assembly is possible. The primary objectives of such a process would be to have a bonding system that would result in higher throughput, higher yields, improved device reliability and effectively lower cost.
To achieve these primary objectives, it is necessary that the bonding system meet the secondary objectives of being adaptable to a highly automated in-line process (eliminating off-line processing altogether) by which the mounted IC chip can be conveyed directly and immediately to wire bonding.
New, high-performance polymers, in ribbon form, are now being used for IC attachment. These polymers offer advantages over conventional gold-silicon eutectic and epoxy, polyimide and glass paste processes.
This new generation of adhesives meets or surpasses the fundamental requirements for adhesion, purity, out-gassing, conductivity, stress and shelf/machine-life. Clear advantages are realized in higher production-line throughput, increased automation, waste reduction, controlled bondline-thickness, re-workability, void-free bond-lines and higher yield due to the elimination of problems associated with handling pastes such as stringing/tailing and improper dispense quantity.